Microencapsulation of materials for transport to and/or growth within an animal is an area of research currently attracting much interest. The use of microcapsules provides the potential for such medically important procedures as treatment of insulin-dependent diabetes mellitus (IDDM) in humans through transplantation of insulin-producing cells, and timed release or long term delivery of drugs to an animal.
The microencapsulation membrane plays a critical role in the treatment of IDDM by microencapsulated islet cells, as well as treatment of other diseases with other encapsulated material. Not only must it prohibit proteins of the immune system from entering the capsule, but it must also interact with the host tissues in a biocompatible way. In this sense, biocompatibility means that the membrane will not initiate an inflammatory response and that it will not support cell adhesion and stimulate overgrowth. If overgrowth occurs, the oxygen and nutrient supply to the islets will be limited and they will die. The area of biocompatibility of microcapsules, however, has received relatively little attention.
The principle of immunoisolation is to surround the cells with a biocompatible semipermeable membrane, which allows free diffusion of nutrients, messenger compounds, cell wastes, and cell products, while isolating the cells from the host's immune system. The cells may be individual or clumped in tissue. Messenger compounds and cell products include glucose, Ca.sup.2+, and insulin.
Two methods for the immunoisolation of cells exists: hollow-fiber devices and microencapsulation. One form of hollow-fiber devices are an artificial capillary system consisting of hollow fibers to which cells are seeded on their exteriors, and which are enclosed in a rigid chamber that is connected to the recipient as a vascular shunt. Early devices using insulin producing cells reversed diabetes for limited periods with high doses of heparin. Sun et al. (1977) Diabetes, 26:1136-39. But even with heparin, blood clot formation was a major problem. To reduce the formation of clots, which suffocated the cells, Altman et al. have seeded Amicon fibers successfully, with nearly half of the animal recipients having normal blood glucose levels for over a year. Altman et al. (1986) Diabetes, 35:625-33. However, the Amicon fibers are fragile and they have a limited surface area available for diffusion. A U-shaped ultrafiltration design developed by Reach et al. can solve the diffusion problem, but this design still suffers from the fragility of the Amicon fibers and from the formation of blood clots at the junction with the vascular system. Reach et al. (1984) Diabetes, 33:752-61.
The transplantation of microencapsulated cells or tissue can overcome the hollow-fiber associated problems of diffusion limitations and vascular complications. Originally, Sun and Lim demonstrated the technique by encapsulating rat pancreatic islet cells into a membrane composed of layers of alginate, polylysine, and polyethyleneimine. Sun and Lim (1980) Science, 210:908-910. The microcapsules were injected into chemically induced diabetic rats. These microcapsules corrected the diabetic state for only 2 to 3 weeks.
Gradually the technique improved. A large improvement was making the multi-layer membrane from alginate-polylysine-alginate, which is stronger and has controllable permeability parameters. Goosen et al. (1985) Biotech. and Bioengin., XXVII:146-50. King and Goosen et al. developed methods for decreasing the viscosity of the gel inside so that the tissue or cells are in a more natural environment (1987) Biotechnology Progress, 3:231-240. A further advance was making the microcapsules in a more uniform, smooth, spherical shape which improved their strength. Walter et al. Poster Group H.
With these changes, Sun et al. have transplanted rat islets of Langerhans into chemically induced diabetic rats which have reversed the diabetic state for up to 780 days, Sun (1987) Trans Am Soc. Artif. Intern Organs, XXXIII:787-790. In vitro studies have shown that antibodies from Type 1 human diabetic patients were not able to suppress encapsulated cells, Darquy and Reach (1985) Diabetologia, 28:776-80. Therefore, it seems that microencapsulation can protect cells from antibodies. However, there are still several serious problems in regards to biocompatibility. Sun has reported finding fibroblast-like cells on the external surfaces of intact microcapsules. Sun had transplanted the microencapsulated rat islet cells into diabetic rats. Sun (1987) Trans Am. Soc. Artif. Intern Organs XXXIII:787-790. In his articles, Sun has specifically recognized the need to improve the biocompatibility of the microcapsules. Id. at 810.
Other published studies have also seen this inflammatory response to transplanted microencapsulated cells. In another study of microencapsulated rat islet cells, O'Shea and Sun found that the microcapsules that they transplanted into chemically induced diabetic mice had cell overgrowth in the range of 0-10 layers of cells. The overgrowing cells included fibroblasts, macrophage-like cells and neutrophils. There was also collagen around the capsules. O'Shea and Sun (1986) Diabetes, 35:943-946. Again, O'Shea and Sun expressly recognized that the biocompatibility of the microcapsules must be improved. Id. at 946.
The inflammatory response is not limited to transplants of islet cells. Wong and Chang have reported recovering clumped microcapsules of rat hepatocytes after they were transplanted into mice with liver failure. They found no viable cells within the clumped microcapsules, which were recovered only seven days after transplantation. Wong and Chang (1988) Biomat., Art. Cells, Art. Org., 16(4):731-739. The cells probably died because they were cut off from nutrients when the cells grew over the semi-permeable membrane.
Current formulations for microcapsules result in algin--polycationic polymer--algin composites. The exterior of these membranes are negatively charged, due to the algin, and may have positive charges due to exposed polycation; as such they support protein adsorption and cell attachment. In general, these microcapsules become overgrown with fibroblasts and other cells. This overgrowth has many negative effects, including impairment of the functioning of the microcapsule by blocking permeability, and induction of immune response by the host animal.
The microencapsulation technique that had previously met with the most success is that of O'Shea and Sun (1986) Diabetes, 35:943-946. Their method uses the strong interaction between large multicharged molecules, one cationic and one anionic, to form a very thin, stable, spherical membrane shell that resists the diffusion of large proteins, such as antibodies, while allowing the diffusion of smaller proteins, such as insulin.
Such a membrane is obtained by suspending cells to be encapsulated in a solution of algin, a polyanionic polysaccharide that is obtained from kelp. Very small droplets of this solution are formed, approximately 0.1-1.0 mm in diameter depending on the size of the material to be encapsulated, and these droplets are gelled on contact with a fairly highly concentrated solution of calcium chloride, 0.2-1.6% CaCl.sub.2. The calcium cations, in this high concentration, serve to reversibly crosslink the anionic polysaccharide chains, forming the gel.
To obtain a membrane that would be stable at physiologic concentrations of calcium, the negatively charged microcapsule is placed in a solution of a positively charged polymer, for example, polylysine. The opposite charges interact, leading to very strong adsorption of the polylysine, resulting in a stable, strongly crosslinked surface. Similarly, an outer layer of algin is added, yielding an algin-polylysine-algin composite trilayer membrane.
The solid inner core of gelled algin is liquefied by placing the microcapsules in a solution of sodium citrate to chelate the gelling calcium. At physiologic calcium levels, the core remains liquid and the algin, if of low enough molecular weight, diffuses out of the core. The result is a spherical shell of algin-polylysine-algin surrounding the microencapsulated cells.
It is predictable that such an algin-polylysine-algin microcapsule will not resist tissue overgrowth. The exterior surface is highly charged, with both positive and negative charges, and would thus be expected to adsorb significant amounts of protein and to support cell adhesion. Andrade et al. (1986) Adv. Polymer Sci., 79:1-63. Experimentally, tissue overgrowth has been observed to be the point of failure of the microencapsulation therapy. O'Shea and Sun (1986).
Poly(ethyleneoxide) (PEO) has been used in numerous instances to decrease cellular attachment. For instance, PEO coatings on PVC tubes have been reported as significantly reducing platelet adhesion in vitro and preventing adhesion and thrombus formation in 72 day PVC tube implants in vivo. (Y. Mori et. al., Trans. Am. Soc. Artif. Intern. Organs 28:459 (1982)). Volume restriction and osmotic repulsion effects were credited with producing the low adsorption of blood constituents. (Id.). Later research concluded that there were micro flows of water induced by the cilia-like movements of hydrated PEO chains which prevent plasma proteins from absorbing onto the surface of coated PVC tubes. (S. Nagaoka and A. Nakao, Biomaterials 11:119 (1990)).
In addition to PVC tube coatings, others have reported that segmented polyurethanes containing PEO as the soft segment, when cast as films or coatings, show reduced platelet retention in vitro. (E. W. Merrill et al., Trans. Am. Soc. Artif. Intern. Organs 28:482 (1982)). PEG-polyurethane coatings on disks made of Pellethane were shown to cause the disks to have reduced cellular adhesion for up to 3 months when implanted into the peritoneal cavities of mice. (S. K. Hunter et al., Trans. Am. Soc. Artif. Intern. Organs 29:250 (1983)). Block co-polymers consisting of poly(N-acetylethyleneimine) and PEO, when coated on solids such as glass beads or silica, were found to increase the homeo-compatibility of the solids by decreasing adsorption of hydrophilic macromolecules. (C. Maechling-Strasser et al., J. of Biomedical Materials Research 23:1385 (1989)).
Long term canine vascular implants of Biomer coated with polymers have been tested for adsorption of proteins. (C. Nojiri et al. Trans. Am. So. Artif. Intern. Organs, 35:357 (1989)). The polymer coatings found to have "excellent non-thrombogenic performance" were: 1) Heparin immobilized on Biomer using a long chain PEO spacer; and 2) a block copolymer composed of 2-hydroxyethyl methacrylate (HEMA) and styrene. (Id.).
Low density polyethylene (a hydrophobic polymer surface) coated with a block copolymer containing water insoluble components (such as polypropylene oxide or polybutylene oxide) and PEO components (water soluble components) was shown to have protein resistant properties. (J. H. Lee et al., J. of Biomedical Materials Research, 23:351 (1989)). The surface was created by a simple coating process where the hydrophobic components of the polymer adsorbed on the hydrophobic surface of the polyethylene from an aqueous solution, the PEO chains were then at least partially extended into the aqueous solution creating a protein resistant surface. (Id.).
Processes used to achieve PEO surfaces other than a simple coating process have been described: block co-polymerization (Y. Mori supra); incorporation into polyurethanes (E. W. Merrill supra); and direct attachment of PEO molecules to the cyanuric chloride activated surface of a poly(ethylene terephthalate) film. (W. R. Gombotz et al., J. of Applied Polymer Science 37:91 (1989)).
Most of the foregoing uses of PEO have been on concave or flat surfaces; they have not been on small convex surfaces such as are found with microcapsules. Due to the fact that the nonionic water soluble polymers face outward from the microcapsule, it could not be predicted from the prior art that PEO and other non-ionic water soluble polymers could form a sufficient barrier to protect microcapsular surfaces. Likewise, the chemistry was not known for attaching sufficient quantities of water soluble non-ionic polymers to the outer surfaces of microcapsules to create this barrier.